Cylinder head porting is the art of reshaping intake and exhaust runners to move more air. But airflow is not linear with port size — get it wrong and you make less power than you started with.

The flow bench basics. A flow bench pulls air through the port at a fixed pressure drop, usually 28 inches of water (about 1 psi). Airflow is measured in CFM at various valve lifts. A stock small-block Chevy head might flow 210 CFM at 0.500" lift; a well-ported version can hit 280 CFM. Numbers are always reported with the valve lift and test depression noted.

Why bigger is not better. Port cross-section controls velocity. Air has mass, and a fast-moving column of air rams itself into the cylinder even after the piston reverses direction (ram effect). Over-porting drops velocity, which:

• Kills low- and mid-range torque (the column loses inertia)
• Reduces fuel atomization in port-injected engines (slow air lets fuel drop out of suspension)
• Delays cylinder filling until very high RPM — often above the engine's useful range

The target: 240–260 ft/sec mean port velocity at peak power RPM. Below 200 ft/sec you lose ram effect; above 300 ft/sec the port chokes.

Rule of thumb — port CSA sizing:

Required CSA (sq. in.) = (CID per cylinder × peak RPM) / (88,200 × target velocity in ft/sec ÷ 240)

Or the practical version: for a 350ci V8 (43.75 ci per cyl) making peak power at 6,500 RPM, target port CSA is roughly 2.2 sq. in. — about the size of a US quarter with some margin. Go bigger and you lose torque everywhere below redline.

Real-world example — the LS3 vs LS7 head trap. Swapping LS7 heads (270 CFM) onto a 6.2L LS3 sounds like free power. In practice, dyno tests routinely show lost torque below 4,500 RPM because the LS7's 260cc intake runners drop velocity in the smaller-displacement engine. The LS3's 260 CFM heads with 257cc runners keep velocity in the sweet spot and make more area under the curve.

Where porting actually pays. The short-side radius (the tight turn just above the valve) is where 70% of your flow gains hide. A skilled porter blends this radius rather than hogging out the whole runner. The bowl area just under the valve seat is the second highest-value zone. Runner walls? Barely matters — smooth is fine, mirror polish is a myth (a slightly rough intake wall actually helps fuel suspension).